Inconel 600 vs Monel 400: Nickel Alloy Selection for Caustic and Marine Applications

Choose Monel 400 (UNS N04400) for caustic soda and for seawater, and Inconel 600 (UNS N06600) for high-temperature oxidation, deaerated high-purity steam and dry chlorine. Monel 400 is a nickel-copper alloy with no chromium: it survives hot sodium hydroxide and flowing seawater at very low corrosion rates because it does not depend on a passive oxide film, and it fails in air above roughly 540 C for the same reason.

Inconel 600 is a nickel-chromium-iron alloy whose 14-17 % chromium builds a tenacious Cr2O3 scale: that scale carries it to 1093 C in oxidation service, but it also leaves the alloy with no answer for hydrofluoric acid. The single condition that defeats both alloys is chlorate contamination in concentrated caustic, where Nickel 200 or 201 becomes the reference material. Establish the stream chemistry before you select the alloy.

Inconel 600 vs Monel 400

Monel 400 is the better default for caustic soda and for marine service, and Inconel 600 is the better default for high-temperature oxidation, oxidizing acids and deaerated high-purity steam. The ranking flips depending on whether the dominant risk in your process is general corrosion in alkali and chloride-bearing water, or heat and cracking in a high-temperature environment.

Rationale: the two alloys are built on different protective mechanisms, and everything else follows from that. Monel 400 relies on the intrinsic nobility of a nickel-copper solid solution - roughly 63-65 % nickel and 28-34 % copper - so its resistance does not collapse when a passive film is broken, but it also has no film to protect it from high-temperature oxidation. Inconel 600 relies on a chromium oxide scale that regenerates in oxidizing conditions, which is why it holds up in air at 1093 C (2000 F) and why it is unusable in reducing acids such as hydrofluoric acid. Identify which mechanism your process attacks, and the choice between them is usually decided in a single step.

Table 1 - Inconel 600 vs Monel 400: headline selection comparison

What Are Inconel 600 and Monel 400, and Why Are They Compared So Often?

Inconel 600 and Monel 400 are two of the oldest and most widely stocked solid-solution nickel alloys, and they are compared constantly because they occupy the same commercial niche: both cost far more than stainless steel, both are far cheaper than the molybdenum-bearing superalloys such as Hastelloy C276, and both are routinely offered in the same mill forms from the same warehouses.

Rationale: when a design team discovers that 316L is failing in a caustic circuit or a seawater line, the first two nickel-alloy quotes they receive are almost always Inconel 600 and Monel 400. The two alloys also have overlapping raw-material costs, which removes price as a tiebreaker and forces the decision to be made on corrosion data and fabrication requirements instead. That is precisely why the comparison is worth doing properly rather than by habit.

What Is Inconel 600 (UNS N06600)?

Inconel 600 is a nickel-chromium-iron alloy with a minimum of 72 % nickel, 14-17 % chromium and 6-10 % iron, standardised as UNS N06600 and Werkstoff 2.4816 (EN NiCr15Fe). It is non-magnetic, cannot be hardened by heat treatment, and is strengthened only by cold work.

Rationale: Inconel 600 was developed for heat rather than for wet corrosion. Its chromium content forms a protective Cr2O3 scale that survives repeated thermal cycling to 1093 C (2000 F), and its high nickel content gives it what suppliers describe as virtual immunity to chloride-ion stress-corrosion cracking. That combination is why the alloy became the standard material for furnace components, nitriding retorts, catalyst regenerators and nuclear steam generator tubing, and why it later found a home in chemical plant duty where high purity and low chloride cracking risk matter more than acid resistance. Mill forms and their individual standards are set out on our Inconel 600 complete guide page, with pipe covered under Inconel 600 pipes and properties and uses of Inconel 600 pipe, and flanges under the Inconel 600 flange guide.

What Is Monel 400 (UNS N04400)?

Monel 400 is a nickel-copper alloy with a minimum of 63 % nickel and 28-34 % copper, standardised as UNS N04400, Werkstoff 2.4360 and EN NiCu30Fe. It is a single-phase face-centred-cubic solid solution with no chromium at all, and it is strengthened by cold work rather than by heat treatment.

Rationale: the absence of chromium is the defining engineering fact about Monel 400. Because it does not depend on a chromium oxide film, it performs well in exactly the environments that destroy passive alloys - flowing seawater, chlorides under deposits, and reducing acids such as hydrofluoric acid - but it has no defence in strongly oxidizing media such as nitric acid or hot air. Its copper content is what makes it the outstanding material for hydrofluoric acid, and its nickel content is what gives it resistance to caustic soda and immunity to chloride SCC. Grade data is collected on the Monel 400 properties and composition page, with applications on applications and advantages of Monel 400 alloy.

How Do the Chemical Compositions of Inconel 600 and Monel 400 Differ?

Inconel 600 is a nickel-chromium alloy with iron, while Monel 400 is a nickel-copper alloy with no chromium. That single structural difference - chromium versus copper as the principal alloying partner - explains almost every performance difference discussed in the rest of this guide.

Chromium drives passivation, which is a surface phenomenon. It buys resistance to oxidizing chemistry and to high-temperature scaling, and it does nothing for reducing chemistry. Copper drives stability in reducing and neutral environments, particularly in hydrofluoric acid and flowing seawater, and it offers no protection whatsoever in oxidizing conditions. Note also that both alloys cap carbon deliberately - 0.15 % maximum in Inconel 600 and 0.30 % maximum in Monel 400 - to limit carbide precipitation at grain boundaries in welded and elevated-temperature service.

Table 2 - Chemical composition, weight percent (ASTM limits)

Which Alloy Should You Choose for Caustic Soda (NaOH) Service?

Monel 400 is the better of the two for caustic soda and caustic potash service across the range that process plants normally operate, because it combines very low general corrosion rates with the lower risk of caustic stress-corrosion cracking. Inconel 600 is a legitimate caustic material and is widely used in the alkali industry, but it carries a documented caustic-cracking risk that has to be managed through stress control rather than ignored.

Rationale: the general corrosion behaviour of the two alloys in caustic is so close that it is almost interchangeable. In a plant test at 23 % NaOH and an average temperature of 104 C, the measured rates were 0.004 mm/y for Nickel 200, 0.005 mm/y for Monel 400 and 0.004 mm/y for Inconel 600 - differences far below the accuracy of the test. What separates them is cracking. Caustic SCC, sometimes called caustic embrittlement, produces sudden intergranular or transgranular failure with almost no metal loss, so it is invisible to a corrosion-allowance calculation and is the failure mode that actually takes caustic equipment out of service.

Table 3 - Caustic soda corrosion rates: Inconel 600 vs Monel 400 vs Nickel 200

Sources: Nickel Development Institute plant and laboratory test data in caustic soda, as compiled in published corrosion-by-alkalies guidance; 1 mpy = 0.0254 mm/y. Values are indicative and should be confirmed against the actual liquor chemistry before specification.

What Is Caustic Stress-Corrosion Cracking, and Which Alloy Cracks First?

Inconel 600 carries the higher caustic SCC risk of the two alloys; Monel 400 resists caustic cracking across most plant conditions and only becomes vulnerable under unusually high stress combined with elevated temperature.

Caustic cracking requires three things at once - a concentrated alkali, a tensile stress, and enough temperature. Inconel 600 has been shown to crack in 50 % caustic at temperatures above roughly 300 C in the presence of air, and in deaerated 10 % NaOH at 315 C under test conditions; laboratory work has also linked its cracking susceptibility to electrochemical potential rather than to concentration alone, with a maximum around 200 mV more positive than the natural corrosion potential.

Oxygen-free conditions suppress cracking substantially, which is why the same alloy can serve for decades in one caustic circuit and crack within months in another. Monel 400 approaches the caustic performance of Nickel 200, and reference guidance notes that it can suffer caustic cracking only under very high stress at elevated temperature.

Controlling caustic SCC is cheaper than changing alloy. Because residual weld stress is one of the three inputs, a stress-relief heat treatment on fabricated joints often allows Inconel 600 to be used in a caustic circuit where it would otherwise be rejected - and the same treatment improves the service life of a Monel 400 or stainless steel system. Specify stress relief deliberately, and say so explicitly in the fabrication scope.

When Should You Choose Nickel 200 or 201 Instead?

Choose commercially pure Nickel 200 or 201 whenever the caustic liquor contains chlorate or hypochlorite contamination, whenever the concentration is above roughly 70 % at high temperature, or where the duty is anhydrous or molten caustic. Nickel 200 and 201 handle all concentrations of caustic soda up to and including the molten state.

Chlorate is an oxidant, and an oxidant destroys the protective film on both Monel 400 and Inconel 600. The published data is stark: adding 0.30 % sodium chlorate to a 73-96 % caustic evaporator liquor moved Nickel 200 from 0.038 mm/y to 6.60 mm/y, and Inconel 600 from 0.056 mm/y to 9.65 mm/y - a swing of two orders of magnitude caused by a trace contaminant.

Because chlorate is a normal by-product of the chlor-alkali process, a caustic evaporator specification that does not state a chlorate limit is an incomplete specification, whichever of the two alloys you originally intended to buy. Our Nickel 200 and Nickel 200 application pages cover the commercially pure nickel alternative in more detail.

Which Alloy Is Better for Marine and Seawater Service?

Monel 400 is the better marine material of the two, and it is the better marine material by a wider margin than the caustic comparison suggests. In flowing seawater its general corrosion rate is below 0.025 mm/y (1 mpy), against roughly 0.05-0.13 mm/y (2-5 mpy) for Inconel 600, and it resists pitting and crevice attack far more reliably in slow-moving or fouled conditions.

Rationale: stainless steels and chromium-bearing alloys such as Inconel 600 protect themselves with a passive film, and that film is vulnerable in seawater. Chloride ions attack it locally, and where oxygen is consumed under a gasket, a barnacle or a sludge deposit, the film cannot repair itself, so the attack concentrates. Monel 400 does not rely on a film: its nickel-copper matrix is thermodynamically stable in seawater, so removing oxygen locally does not create a weak point.

That is why Monel 400 has served for decades in propeller shafts, seawater valve trim, pump impellers and tidal-zone fasteners, and why the alloy acquired its reputation as a marine benchmark. For the wider material shortlist, see which alloy to use for seawater piping and our marine engineering hub, which also covers cost-driven alternatives such as super duplex S32750 for seawater desalination.

Table 4 - Seawater performance: Monel 400 vs Inconel 600, with 316L for reference

Do not select on general corrosion rate alone in seawater. Both alloys look acceptable on a uniform-rate table, and stainless steels often look acceptable too. The failures that actually happen in seawater piping are localized - crevice attack under flanges, pitting under fouling, and erosion-corrosion at high-velocity bends - so the specification should address geometry and velocity as well as alloy grade.

How Do Inconel 600 and Monel 400 Compare in Mechanical and Physical Properties?

Inconel 600 is the stronger and stiffer of the two in the annealed condition and holds that advantage far better at temperature. Monel 400 is softer and less strong at room temperature but is more thermally conductive, which makes it the better choice for heat-transfer equipment in seawater and caustic duty.

Both alloys are solid-solution strengthened and cannot be precipitation-hardened, so their room-temperature properties are similar in kind if not in level. The divergence appears at temperature. Inconel 600's chromium-bearing matrix retains strength to high temperatures, which is why it appeared in furnace and nuclear applications, while Monel 400 loses strength as the temperature rises and is ultimately limited by oxidation rather than by creep. Thermal conductivity cuts the other way: Monel 400 conducts roughly 47 % more heat than Inconel 600 at 100 C, which translates directly into less surface area in a heat exchanger.

Table 5 - Mechanical properties, annealed condition (typical and specified minimums)

What Are the Temperature Limits of Inconel 600 and Monel 400?

Inconel 600 is serviceable in air to 1093 C (2000 F) and can carry structural load to roughly 816 C (1500 F). Monel 400 is limited to about 480 C (900 F) in continuous air service, with rapid oxidation setting in above roughly 540 C (1000 F). The gap is a factor of roughly two, and it is entirely attributable to chromium.

Rationale: high-temperature oxidation is a scale-growth problem, and scale growth requires an element that forms a slow-growing, adherent oxide. Inconel 600 has 14-17 % chromium, which produces a Cr2O3 layer that stays protective and reheals after thermal cycling; above 1093 C the oxide begins to volatilise as CrO3 gas and recession accelerates. Monel 400 has no chromium, so it forms a fast-growing, poorly adherent nickel-copper oxide that spalls and allows continued metal loss. This is also why Monel 400 is excellent at cryogenic temperatures - the same absence of a brittle film - and it remains ductile to -100 C and beyond.

Table 6 - Temperature limits and service envelopes

Which Alloy Performs Better in Acids and Other Corrosive Media?

Monel 400 wins in reducing acids, and Inconel 600 wins in oxidizing acids. There is no general answer: the acid has to be named, along with its concentration, temperature, aeration and chloride content, before either alloy can be recommended.

An oxidizing acid supplies the electrons that keep a chromium oxide film intact, so it rewards Inconel 600. A reducing acid removes them, so a passive film cannot stabilise and the alloy must rely on the intrinsic corrosion resistance of its matrix - which is where the copper in Monel 400 earns its place. The distinction matters in practice because process streams change character along a flowsheet: a deaerated acid feed behaves as a reducing medium, while the same acid after air sparging or after contacting an oxidant behaves as an oxidizing one. Selection must follow the chemistry at the metal surface. For a full medium-by-medium screening method, see selecting nickel alloy pipe for chemical plants by acid type.

Table 7 - Corrosion behaviour by medium (indicative rates where published)

Rate values are representative figures compiled from published nickel-alloy corrosion data (1 mpy = 0.0254 mm/y). Actual rates depend on concentration, temperature, aeration, velocity and trace contaminants. Treat this table as a screening tool, not a substitute for project-specific testing.

How Do the Two Alloys Behave in High-Temperature Oxidation and Carburization?

Inconel 600 resists oxidation and carburization well enough to serve as a furnace material; Monel 400 should not be used in hot oxidizing or carburizing atmospheres at all. This is the clearest single dividing line between the two alloys.

Rationale: carburization is the inward diffusion of carbon, which raises surface hardness, embrittles the material and eventually destroys ductility. It is driven by carbon activity in the gas and resisted by a stable surface oxide that blocks carbon transport; Inconel 600 has that oxide and Monel 400 does not. In hydrogen plus 2 % methane carburizing tests, Inconel 600 gained 2.66 mg/cm2 at 925 C and 12.30 mg/cm2 at 1095 C over 100 hours, a manageable rate. Note also that Inconel 600's weakness is sulfur: at high temperature in strongly sulfur-bearing gas it sulfidizes, and a higher-chromium alloy should be considered instead.

How Are Inconel 600 and Monel 400 Welded, and Which Filler Metals Apply?

Both alloys weld readily with the GTAW, SMAW and GMAW processes, but each requires its own nickel-base filler metal. Use ERNiCr-3 bare wire or ENiCrFe-3 covered electrodes for Inconel 600, and ERNiCu-7 bare wire or ENiCu-7 covered electrodes for Monel 400. Never weld either alloy with a steel filler.

Matching the filler to the base metal keeps the weld pool chemistry inside the range where it solidifies with adequate ductility and corrosion resistance. Two failure modes dominate shop mistakes. First, sulfur and lead contamination: both alloys are sensitive to these elements, and sulfur-bearing marking crayons, cutting fluids and oily shop rags are common sources. Second, iron dilution in Monel 400 welds, which pushes the deposit toward a nickel-copper-iron composition with poorer solidification behaviour. Both risks are managed by cleaning and by controlling dilution rather than by exotic consumables. Procedure-level detail for the wider nickel alloy family is available in our Inconel 625 welding guide and Hastelloy C276 welding guide.

Table 8 - Welding and fabrication comparison

What Do Inconel 600 and Monel 400 Cost, and How Should You Justify the Premium?

The two alloys sit in the same commercial band, and neither is reliably cheaper. Indicative mid-2026 ranges for plate and sheet are roughly USD 25-38/kg for Inconel 600 and USD 28-60/kg for Monel 400, with bar and heavy sections priced higher and regional delivered pricing higher again. Do not select between them on price.

Rationale: it is worth understanding why they cost about the same, because the reason matters for budgeting. Nickel makes up about 72 % of Inconel 600 by weight and about 63-65 % of Monel 400, so the raw-material burden is comparable - Monel 400 offsets its lower nickel content with roughly 30 % copper, and the two metals frequently trade within a factor of two of each other on a per-tonne basis. Because nickel alone drives 55-70 % of a nickel alloy's raw-material cost, both grades move together when the LME nickel price moves. The engineering consequence is that the alloy choice should be made on corrosion and fabrication logic, and the money-saving conversation should be about substitution against stainless steel or super duplex, not between these two.

Table 9 - Indicative 2026 cost comparison (budgeting reference, not a quotation)

Which ASTM, ASME and Equivalent Standards Apply to Each Alloy?

Inconel 600 and Monel 400 use parallel but entirely separate standard series. Inconel 600 is covered by ASTM B166, B167, B168, B516, B517, B564 and B366; Monel 400 is covered by ASTM B127, B163, B164, B165, B474, B730, B564 and B366. Both are recognised under ASME with an SB prefix and carry distinct P-Numbers for welding qualification.

Specifying the wrong product-form standard is one of the most common causes of a rejected delivery, because the mechanical property minimums differ between forms of the same alloy. A Monel 400 seamless heat-exchanger tube ordered to ASTM B163 does not carry the same requirements as Monel 400 pipe ordered to B165, even though both are UNS N04400 and both may arrive from the same mill. State the alloy, the UNS number, the product form and the standard together, every time.

Table 10 - Product forms and applicable standards

Table 11 - Equivalent and cross-reference designations

What Are the Typical Applications of Each Alloy in Caustic and Marine Service?

Monel 400 dominates caustic evaporation, seawater handling and hydrofluoric acid alkylation service, while Inconel 600 dominates furnace and thermal-processing hardware, high-purity and nuclear steam systems, and chemical processes that are oxidizing or that require freedom from chloride cracking.

Applications follow the mechanism. Where a plant is handling alkali and salt water, the material must survive without a passive film, so it is a nickel-copper alloy. Where a plant is generating heat, handling oxidizing chemistry, or operating a high-purity steam cycle, the material must maintain a stable oxide, so it is a nickel-chromium alloy. The practical value of this framing is that it lets a procurement engineer predict which of the two a process licensor is likely to have specified, and therefore which spare parts and which filler metals need to be stocked. Product-form pages for the most commonly ordered items are Monel 400 pipe, Monel 400 sheet and plate, Monel 400 round bars and ASTM B564 Monel 400 flanges.

Table 12 - Application mapping by industry and duty

How Do You Run an Inconel 600 vs Monel 400 Selection Decision in Seven Steps?

Run the selection as a seven-step procedure that starts with the process chemistry and ends with material verification. The steps are ordered deliberately: the first three eliminate the wrong alloy on physics, the next three test the surviving candidate against fabrication and system constraints, and the last one protects the decision during procurement and construction.

Most material failures attributed to the wrong alloy are not alloy failures at all. They are specification failures - a chlorate limit that was never written down, a steam-out temperature that was never listed, a stress-relief requirement that was dropped to save schedule. A short, ordered procedure catches those omissions before the purchase order is cut, when a change costs nothing.

The seven steps in order

1. Define the stream chemistry, not the alloy family - Write down the actual service medium with numbers: NaOH or KOH concentration in wt %, chloride content in ppm, chlorate content in ppm, dissolved oxygen, pH, and whether the stream is oxidizing or reducing. Chloride and chlorate are the two contaminants that most often overturn a selection made on alkali concentration alone.
2. Fix the full temperature envelope - Record steady-state, upset, standby, steam-out and cleaning-cycle temperatures. Caustic stress-corrosion cracking is driven by temperature and residual stress together, so a unit that runs cold but is steam-cleaned at 150 C can still crack.
3. Set an acceptable corrosion allowance - Choose a design corrosion rate - 0.1 mm/y (about 4 mpy) is a common ceiling for pressure-containing process equipment - then screen both alloys against published iso-corrosion data. Both alloys pass this screen in most caustic and seawater duties; the screen exists to disqualify them in the minority of cases where it fails.
4. Assess cracking risk separately from general corrosion - Caustic SCC and chloride SCC rank these two alloys differently from general corrosion. In caustic, Monel 400 carries the lower cracking risk. In chloride-bearing water and high-purity deaerated steam, Inconel 600 is the stronger performer. Never assume the alloy that thins slowest also cracks last.
5. Confirm fabrication, welding and stress relief - Check filler metal availability in the project region, whether the joint is a dissimilar weld, and whether a stress-relief treatment will be specified. Residual weld stress is a controllable input to caustic SCC and is often the cheapest risk reduction available.
6. Check the non-corrosion requirements - Verify high-temperature strength and creep life, thermal conductivity for heat-transfer duty, magnetic signature if the part sits near instrumentation or minesweeping equipment, and NACE MR0175 / ISO 15156 coverage if the stream is sour.
7. Verify the delivered material against the specification - Confirm chemistry by positive material identification (PMI) and review the mill certificate against the correct product-form standard. For caustic service, add a chlorate limit to the purchase specification rather than discovering it in the process stream.

What Specification Mistakes Cause the Most Failures with These Two Alloys?

Five mistakes account for most of the avoidable failures: omitting the chlorate and chloride limits from a caustic specification, omitting steam-out and cleaning temperatures from the design envelope, assuming that because both alloys resist chloride SCC they are equally resistant to caustic SCC, dropping stress-relief heat treatment to save schedule, and welding with a mismatched filler metal or with sulfur-contaminated shop tooling.

Rationale: each mistake shares a common structure - it removes an input that the corrosion mechanism depends on. Caustic SCC needs temperature, stress and concentrated alkali; specifying a chlorate limit addresses chemistry, stress relief addresses stress, and a full temperature envelope addresses temperature. Remove any one input and the failure mode recedes; leave one unaddressed and it may not appear for years, which is precisely what makes these mistakes expensive and easy to repeat.

Purchase specification checklist for caustic and marine duty: alloy grade and UNS number; product form standard; NaOH or KOH concentration in wt %; chloride limit in ppm; chlorate limit in ppm; maximum and steam-out temperatures; corrosion allowance and design rate; required stress relief; filler metal and welding procedure specification; PMI requirement; and NACE MR0175 coverage if the stream is sour. Attach it to the enquiry, not to the order.

Related Reading and Further Alloy Comparisons

The articles below extend this comparison into adjacent selection decisions. Read them together for a complete picture of where Monel 400, Inconel 600 and the neighbouring nickel alloys each belong.

Inconel 625 vs Monel 400 - how the molybdenum-bearing grade changes the seawater and sour gas picture.

Inconel 625 vs Hastelloy C276 for seawater - the higher-alloy tier above these two grades.

Inconel 625 vs Hastelloy C276 complete selection guide - a full side-by-side method for the next step up in corrosion resistance.

Hastelloy C276 for acid service - where to go when neither Inconel 600 nor Monel 400 survives the medium.

Hastelloy C276 for chemical plants - the mixed-acid and chloride-heavy duty case.

Chemical equipment materials - a hub for vessels, columns and heat exchangers in corrosive service.

Marine engineering materials - seawater, splash-zone and subsea selection in one place.

Inconel 625 for marine engineering - the high-strength marine option.

Nickel alloy FAQ library - quick answers across the full grade range.

For a direct quotation on Inconel 600 (UNS N06600) or Monel 400 (UNS N04400) in plate, sheet, pipe, tube, bar, forgings or fittings, send the grade, product form, standard, size and quantity to the JN Alloy team and we will return a quotation with the mill certificate scope stated.

Frequently Asked Questions

Is Monel 400 better than Inconel 600 for caustic soda?

Yes, for most caustic service. Monel 400 has a comparable general corrosion rate - 0.005 mm/y against 0.004 mm/y for Inconel 600 in a plant test at 23 % NaOH and 104 C - but a lower risk of caustic stress-corrosion cracking, which is the failure mode that actually takes caustic equipment out of service. Choose Inconel 600 for caustic duty only where another requirement, such as high-temperature strength or a sulfur-bearing stream, outweighs the cracking risk, and then specify stress relief.

Can Inconel 600 be used in caustic soda service?

Yes, and it is widely used in the alkali industry, but it is not immune to caustic cracking. Inconel 600 has been shown to crack in 50 % caustic above roughly 300 C in air and in deaerated 10 % NaOH at 315 C under test conditions. Oxygen-free conditions suppress cracking substantially, and stress relief of fabricated joints further reduces the risk. Specify it with those controls in place, not as a drop-in substitute for Monel 400.

Which alloy resists seawater better, Monel 400 or Inconel 600?

Monel 400. Its general corrosion rate in flowing seawater is below 0.025 mm/y (1 mpy), compared with roughly 0.05-0.13 mm/y (2-5 mpy) for Inconel 600, and it resists pitting and crevice attack far better in slow-moving or fouled conditions because it does not depend on a chromium oxide film that chlorides can break down locally.

Is Monel 400 immune to stress-corrosion cracking?

It is treated as virtually immune to chloride stress-corrosion cracking, which is the cracking mode that destroys 304 and 316 stainless steels in hot chloride service. It is not immune to caustic cracking: reference guidance notes that under very high stress and elevated temperature, Monel 400 can suffer caustic cracking, although it approaches the resistance of Nickel 200 and outperforms Inconel 600 in this duty.

Is Inconel 600 resistant to chloride stress-corrosion cracking?

Yes. Inconel 600 is considered virtually immune to chloride-ion stress-corrosion cracking, even in hot and concentrated chloride solutions. This immunity comes from its high nickel content rather than from its chromium, and it is the main reason the alloy was adopted for nuclear steam generator tubing and for high-purity water systems.

What is the maximum service temperature of Monel 400?

About 480 C (900 F) in continuous air service. Because Monel 400 contains no chromium, it forms a fast-growing, poorly adherent oxide, and rapid scaling begins above roughly 540 C (1000 F). The limit is an oxidation limit, not a melting limit; the alloy's melting range is approximately 1300-1350 C.

What is the maximum service temperature of Inconel 600?

Inconel 600 resists oxidation and scaling in air to 1093 C (2000 F). For load-bearing structural service the practical limit is lower - allowable stresses are tabulated to roughly 816 C (1500 F) for standard product forms, and creep becomes the governing design condition above about 650-700 C.

Can Monel 400 be used in hydrofluoric acid service?

Yes, and it is the reference material for the duty. Monel 400 resists hydrofluoric acid at all concentrations to boiling point, including anhydrous HF and HF vapour, with corrosion rates below 1 mpy (0.025 mm/y) in 48 % HF at ambient temperature. This is why HF alkylation units are built around Monel 400 vessels, piping and weld overlays.

Can Inconel 600 be used in hydrofluoric acid service?

No. Inconel 600 is rapidly attacked by hydrofluoric acid because its protective chromium oxide film cannot remain stable in a reducing acid. It should not be specified for HF service at any concentration. Select Monel 400, and for very aggressive HF duties consider a nickel-molybdenum alloy.

Which alloy should be used for nitric acid?

Inconel 600. Nitric acid is strongly oxidizing, which stabilises the chromium oxide film on Inconel 600 and gives it good corrosion resistance. Monel 400 is not recommended in oxidizing acids at all, because its nickel-copper matrix cannot form a protective passive film and it corrodes rapidly and uniformly.

What filler metal is used to weld Inconel 600?

ERNiCr-3 bare wire for GTAW (also designated Inconel 82) and ENiCrFe-3 covered electrodes for SMAW (Inconel 182), both to AWS A5.14 and A5.11 respectively. Inconel 600 is ASME P-No. 43, and it is frequently welded to carbon steel using ERNiCr-3 to bridge P-No. 43 and P-No. 1.

What filler metal is used to weld Monel 400?

ERNiCu-7 bare wire for GTAW (Monel 60 filler) and ENiCu-7 covered electrodes for SMAW (Monel 190), to AWS A5.14 and A5.11. Monel 400 is ASME P-No. 42. Never use a steel filler, and control iron dilution from steel tooling, because iron pickup degrades the weld deposit.

Is Monel 400 magnetic?

Monel 400 is treated as essentially non-magnetic at all temperatures. A slight ferromagnetic response can occasionally be measured after heavy cold work or where iron contamination has been picked up on the surface, so components for magnetic-sensitive duties should be verified rather than assumed.

Is Inconel 600 magnetic?

No. Inconel 600 is non-magnetic in all conditions and at all normal service temperatures, with a Curie point well below ambient. It is used near sensitive instrumentation and in equipment where a magnetic signature would be a problem.

Can Monel 400 and Inconel 600 be welded to each other or to carbon steel?

Yes. Both are welded to carbon steel routinely, using a nickel-base filler that accommodates the difference in expansion and composition - ERNiCr-3 for Inconel 600 and ERNiCu-7 for Monel 400. Welding Monel 400 to carbon steel requires control of iron dilution, and a nickel interlayer is often preferred. For joints between the two alloys, or between either alloy and stainless steel, the welding procedure should be qualified specifically for that combination rather than derived from a base-metal procedure.

Which alloy has better thermal conductivity?

Monel 400, by a wide margin - about 21.8 W/m/K at 100 C against 14.8 W/m/K for Inconel 600, an advantage of roughly 47 %. For heat-transfer equipment handling seawater or caustic, that difference translates into less required surface area and often a smaller, cheaper exchanger.

Which is more expensive, Inconel 600 or Monel 400?

They are close enough that price should not decide the choice. Indicative mid-2026 ranges for plate and sheet are roughly USD 25-38/kg for Inconel 600 and USD 28-60/kg for Monel 400, with both grades moving with the LME nickel price because nickel accounts for 55-70 % of a nickel alloy's raw-material cost. Compare delivered cost including certification, and select on corrosion and fabrication logic.

What happens if the caustic stream contains chlorate?

Both alloys fail, and the specification must escalate to Nickel 200 or 201. Adding 0.30 % sodium chlorate to a 73-96 % caustic evaporator liquor raised the corrosion rate of Nickel 200 from 0.038 mm/y to 6.60 mm/y and that of Inconel 600 from 0.056 mm/y to 9.65 mm/y. Because chlorate is a normal by-product of the chlor-alkali process, a caustic specification without a chlorate limit is incomplete.

Do Inconel 600 or Monel 400 require post-weld heat treatment?

Neither alloy normally requires post-weld heat treatment for corrosion service. However, stress relief is frequently specified where a joint will see hot concentrated caustic, because residual weld stress is one of the three inputs to caustic stress-corrosion cracking. Specifying stress relief is usually the cheapest available risk reduction for caustic duty.

Are Inconel 600 and Monel 400 suitable for sour service under NACE MR0175?

Coverage is limited and must be verified against the specific standard edition and the intended hardness and heat-treatment condition. Monel 400 has limited NACE MR0175 sour-service coverage, and Inconel 600 is not automatically acceptable for all sour duties. Neither alloy should be specified for H2S-containing service without an explicit compliance statement against NACE MR0175 / ISO 15156.

What is the difference between Monel 400 and Monel K-500?

Monel K-500 is the age-hardenable version of the same nickel-copper family, with aluminium and titanium additions that allow precipitation hardening. It offers substantially higher strength and hardness than Monel 400 while retaining the same seawater and caustic corrosion resistance, and it is used for pump shafts, valve stems and fasteners where strength rather than corrosion is the governing requirement.

Can Inconel 600 be used in direct seawater contact?

It can, with care. Inconel 600 gives good general corrosion performance in flowing seawater and is effectively immune to chloride SCC, but it is more prone than Monel 400 to crevice corrosion and pitting in slow-moving, stagnant or fouled conditions. Where geometry is unavoidable - flange faces, gasket seats, under-deposit areas - Monel 400 or a super duplex stainless steel is usually the better choice.